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Rebuild Your Nitro Engine

Re-Power Your Old Engine for Peak Perfomance

How To: Rebuild Your Nitro Engine

At the end of last year’s flying season my nitro engines seemed to be lacking the power I was used to. It was hot and humid, the air was thin and my helicopters were not performing like I like them to. In the end I think I pushed them a bit too hard. My O.S. 91HZ-R went down with a lean run, cooking a small part of the piston and both of my 50-sized engines just weren’t putting out like they used to. They were both getting louder; a sure sign that it is time for new bearings.

The good news, fixing your engine is not a difficult task and once the bearings, piston, ring and sleeve are replaced; your engine will perform just like new for a fraction of the cost of replacing the engine.

Bearings are the most common replacement and for most rebuilds replacing the bearings is all that’s required. The next most common replacement that will add new life to your engine is simply replacing the ring. Finally, if your engine has been run lean or has ingested some grit that has scored the piston and sleeve, replacing the bearings, sleeve, piston, piston ring and wrist pin retaining clips may be necessary.

Other easy replacements include the crankshaft, and connecting rod. Crankshafts can be bent in crashes requiring replacement and connecting rods can be bent or broken in today’s high revving engines.

The first step is to remove your engine from your helicopter and to remove the clutch. I’ll leave the fan hub on at this point.

Next, remove the cylinder head using a 2.5mm driver. I like to store all of the engine parts on a paper towel in a place that I won’t lose them. Place the six head bolts, the head and the head gasket and shim there now.

Remove the back plate using a 2.5mm driver, store the four bolts and back plate on the paper towel with your other engine parts. Using your index finger, reach inside the engine to push the cylinder liner out of the engine. If the cylinder liner is stuck as often happens with well used engines, you can put the corner of a soft towel in the exhaust port and turn the crank shaft gently. The pressure from the piston pressing on the exhaust port of the sleeve will lift the sleeve out so you can pull it the rest of the way.

How To: Rebuild Your Nitro Engine
You’ll need: Vise, glow plug wrench, needle nose pliers, 12mm ratchet, metric Allen wrenches, crank shaft lock, and a toaster oven.

With the cylinder liner completely removed you can now remove the piston. Hook the bottom of the connecting rod with your index finger and pull the connecting rod off the crankshaft. You may need to gently turn the crankshaft back and forth near top dead center to help slide the connecting rod off.

Now with the piston and sleeve removed, you can inspect the damage. There is no question; this engine needs a new sleeve and piston. But, if the piston wasn’t damaged and the sleeve badly scored, I would likely just replace the ring and clean up the sleeve with some scotch bright before reassembly.

Next, remove the crankshaft by loosening the crankshaft nut and fan hub. This is one step where a crankshaft lock is indispensable. Depending on how the fan hub is installed, a bit of heat on the hub can be helpful, but be careful not to melt your cooling fan!

The crankcase is now ready for the toaster oven. If you don’t have a toaster oven, you can use the oven in your kitchen, but be prepared to hear it from your wife when you stink up the kitchen with oil smoke from your crankcase. My toaster oven cost me $40.00, and it was worth it!

Bake the crankcase at 300 degrees for 20 minutes to remove the bearings.

While the crankcase is heating, I like to prepare the replacement parts for installation. First I prep the crankshaft by cleaning it. I’ll soak it in some fuel and then wipe it down with paper towels.

How To: Rebuild Your Nitro Engine
Replacement head, ring, piston, sleeve, ceramic bearings and check-valve. I ordered the check-valve as I thought it may have failed causing the lean run, but as it turns out it was a clogged fuel filter that led to the trashed engine. Lean run’s that result in piston failure tend to pit the head badly, be prepared to replace the head if your piston fails. Bummer, I should have changed that fuel filter more often.

While it soaks, I prep the bearings. I am replacing the stock bearings with high-performance ceramic bearings from Boca Bearings. According to the manufacturer, they can handle more RPM’s and heat better than steel. They are considerably more expensive than steel bearings but I am hoping they will be more durable and provide a longer service life than traditional bearings. I guess time will tell. The front bearing features a pair of rubber seals which the manufacturer recommends leaving in place. The rear bearing also has a pair of rubber seals but the manufacturer recommends removing the seal that faces forward and to leave the seal that faces to the rear to protect the engine in case of a bearing failure.

I then press the rear bearing on to the crankshaft and then place the front bearing on the crankshaft.

Next, prepare the piston by removing the wrist pin clips from the old piston using needle nose pliers. A quick twist and gentle pull is all it takes. I discard the old wrist pin clips and use new ones as they are inexpensive and if the old ones fail, you’ll have to replace the piston and liner again!

Push the wrist pin out of the old piston to remove the connecting rod. Clean both the rod and wrist pin and install in the new piston using new wrist pins.

Install the new ring on the piston. To install the piston ring line up the gap with the index pin on the piston then walk the ring into place by gently pressing on ring and rotating the piston.

Once the crankcase is finished heating, grab it using an oven mitt and firmly tap the back of the crank case on a wood table or wood block. The rear bearing should fall out after one or two firm raps. Next, press the front bearing out of the crankcase. I use an old mainshaft to push it out from the rear of the crankshaft.

After the crankcase cools, clean it as well as you can. Pay particular attention to the areas where the bearings will be seated.

These tend to be the dirtiest but they are also the most critical. Proper cleaning will help ensure the new bearings seat properly.

Once clean, it’s back into the oven for 20 minutes at 300 degrees.

Once heated, remove the crankcase using an oven mitt. First, install the front bearing by sliding it into place using the crankshaft. Then slide the crankshaft out and push it into the crankcase from the rear. I like to use the crankcase lock to push the crankshaft and rear bearing into place. Next, place the thrust washer on the crankshaft, install the fan hub and tighten it down using the crankshaft nut and ratchet.

Once it is fully tightened, rap the front of the crankshaft on a wood block or on your wood table. This will ensure that both the front and rear bearings are fully seated. Then allow the assembly to fully cool.

Reinstall the carburetor and then install the piston by pressing the connecting rod onto the crankshaft. The piston can be installed two ways, the right way and the wrong way! You’ll note there is a large cutout on one side of the piston and a small cutout on the other. Be sure the larger cut out is toward the front of the engine. Also, be careful during this step to avoid scratching the piston.

Next, install the sleeve. Before doing so, double check to ensure that the piston ring is aligned with the index pin on the piston. If it is not lined up, forcing the sleeve into the crankcase will damage the piston and the ring.

Slide the sleeve into the crankcase with the notch on the sleeve facing toward the front of the engine. Use your index finger to help the piston to align with the sleeve. I’ve found once the piston is aligned, simultaneously pushing the sleeve gently and gently wiggling the crankshaft with the fan hub will help the ring compress and seat allowing the sleeve to slide into place. Ensure it is fully seated and aligned with the alignment pin on the crankcase.

How To: Rebuild Your Nitro Engine
Tighten the head bolts like you would the lug nuts on a car tire.

Install the head gasket, shim and cylinder head. I am running 30-percent nitro so I am using both a .2 mm head gasket and .1mm shim. Doing so keeps the engine from detonating and makes the needle-valve setting a little less critical. The goal is simple, align the two gaskets and cylinder head perfectly with the cylinder liner, not hard to do but take your time to ensure a perfect fit.

Next, tighten the six head bolts. I do this in a similar manner to tightening a tire on a car. First I tighten one bolt until it is just barely tight then I go to the bolt on the opposite side of the head, tighten that, then go to the opposite side and tighten the next bolt and so on until all six are lightly tightened. I’ll then tighten each bolt a bit more in the same alternating pattern until the bolts are tight. Some people use a torque wrench for this step. I don’t have one so I just tight them until they are equally snug. We want them tight but don’t get carried away, you don’t need to make them too tight and you certainly don’t want to risk stripping your crankcase by getting carried away.

Replace the back plate and install a new glow plug and your ready to reinstall the engine in your helicopter.

If your engines aren’t delivering the power they used to consider rebuilding them. You’ll be rewarded with all the power of a new engine for a fraction of the cost. Just take your time to be sure to do it right and you will be back in the air in no time.


Words: David Miles